1. Field of the Invention
This invention relates to a process for gasification of cellulosic biomass, such as plants and trees, in a single gasification vessel to produce low and medium Btu fuel gas, that is, gases varying in composition from those rich in CO, H.sub.2, and N.sub.2 to those rich in CO, H.sub.2 and CH.sub.4, respectively. The medium Btu fuel gas may be readily treated to form high Btu substitute natural gas, it can be processed as synthesis gas for methanol production, or may be directly used as an industrial fuel gas. Low Btu gas may be used for direct combustion. The process utilizes readily available and renewable energy resources which are converted by the process of this invention to clean and energy-intensive substitute fuel gases and for use as raw materials for desired chemicals or fuels.
2. Description of the Prior Art
Gasification of carbonaceous materials has been recognized in the past, as for example in U.S. Pat No. 3,884,649 relating to coal gasification, U.S. Pat. Nos. 3,929,715, 3,891,403 relating to gasification of shale, and U.S. Pat. No. 4,263,125 relating to gasification of peat. Gasification of carbonaceous solids, such as coal and shale, is taught by Mitchell et al, U.S. Pat. No. 4,157,245 to be carried out in countercurrent plug flow of carbonaceous material and solid heat transfer material to impede vertical backmixing throughout the gasification zone, maintaining preferably a 500.degree. to 2000.degree. F. temperature difference in the vertical height of the gasifier. This process also requires a separate combustor vessel for heating the heat transfer material and requires complicated solids transport. The Gornowski et al, U.S. Pat. No. 2,633,416 teaches production of water-gas mixtures of carbon monoxide and hydrogen and likewise involves countercurrent plug flow in two or more fluidized beds requiring solids transport. Franke et al, Federal Republic of Germany, Patent Publication No. 2741805 teaches control of the temperature profile along the axis of a reactor for gasification of solid fuels with oxygen-containing gas in a fluidized bed reactor by introduction of reactants at a number of locations along the height of the reactor, most of the fuel being fed to the headspace to fall onto the fluidized bed.
There have been several attempts to gasify biomass. Battelle Columbus Laboratory, Columbus, Oh., has developed a process for gasifying forest residues based upon the Battelle multiple solid fluidized bed gasification process operating at temperatures up to about 1600.degree. F. and atmospheric pressure. The process is described in Feldman, H. F., "Conversion of Forest Residues to Produce a Methane-Rich Gas", paper presented at the Twelfth Biomas Thermochemical Conversion Contractor's meeting, Washington, D.C., Mar. 18, 1981 and utilizes gasifiers similar to the process described in Burton, R. S., and Bailie, R. C., "Fluid Bed Pyrolysis of Solid Waste Materials", Combustion, 13-18 (1974) February, to pyrolyze a variety of biomass materials and animal wastes. The process uses two fluidized bed reactors making up a gasifier and a combustor, thereby requiring transport of solids between the two reactors.
Battelle Pacific Northwest Laboratory has developed a multiple catalytic gasification process for biomass utilizing catalysts, such as potassium carbonate and Ni-Co-Mo, to promote the steam-carbon and hydrocarbon reforming reactions as described in Mudge, L. K. et al, "Progress of Catalyzed Steam Gasification of Biomass", paper presented at the Twelfth Biomass Thermochemical Conversion Contractors meeting, Washington, D.C., Mar. 18, 1981. This process utilizes 1 to 2 percent by weight of catalyst, mixed with the raw biomass and requires a catalyst recovery step. The process employs a fluidized bed gasifier with the energy required for gasification supplied by superheated steam, external electrical heatup, or recycled preheated product gas.
Another gasification process of biomass has been conducted by Wright-Malta Corporation using a pressurized rotary kiln gasifier as described in Coffman, J. A., "Pressurized Steam Gasification of Biomass", paper presented at the Twelfth Biomass Themochemical Conversion Contractors meeting, Washington, D.C., Mar. 18, 1981. This process utilizes alkaline earth metal catalysts to gasify biomass with steam to produce a medium Btu gas.
Another biomass gasification process has been carried out of the Garrett Energy Research and Engineering Co., Inc. involving pyrolysis and combustion of biomass in an atmospheric-pressure multiple-hearth reactor as described in Garrett, D., "Conversion of Biomass to Gaseous Products", paper presented at the Second Annual Fuels from Biomass Symposium, Troy, N.Y., June 20-22, 1978.
The applicants are also aware of fluidized bed biomass gasification being conducted at the University of Missouri, Texas Technical University and Saskatchewan Power Corporation, Regina, Saskatchewan, Canada, all using steam and air for low Btu gasification and the Omnifuel, Canada, fluidized bed oxygen blown gasification process operated at low pressures. Some of these processes utilize freeboard feeding and, in some cases, external heating is provided.
Biomass and solid wastes have also been used in gas production utilizing a fluidized bed pyrolysis system as described in Epstein, E., Kosstrin, H. and Alpert, J., "Potential Energy Production in Rural Communities from Biomass and Wastes using a Fluidized-Bed Pyrolysis System", Energy Resources Co., Inc., Cambridge, Mass., paper presented at Symposium on Energy from Biomass and Wastes, IV, Washington, D.C., Aug. 14, 1978. The Epstein article teaches gas yields of up to about 50 to 60 percent from corncobs while yielding char in the order of 10 to 20 percent.
The extent of the prior work indicates a clear need for an effective single stage, non-catalytic, pressurized, fluidized bed gasification process wherein a large percentage of methane gas is produced directly from the biomass material while producing very low quantities of biomass ash so that the ash can be removed in the gas stream. A process which does not require external heating or require solids circulation between reactant stages is particularly desired.